Among cables that can be used in water, conductor or copper wire of some cables is covered with an insulation portion that is made of polymer material and the like. As the polymer material, for example, thermosetting resin, such as cross-linked polyethylene which will be referred to as XLPE, is used in many cases.
The insulating cables that are laid in water include those used to propagate inverter voltage (AC voltage) and the like. Such an insulating cable could suffer water trees emerging in its insulating portion, such as XLPE, after several years of its use with AC voltage, including inverter surges. The water trees is a kind of an insulation deterioration phenomenon that occurs in the insulating material of the insulating portion, which is triggered by an electric field being applied to the insulating portion in the case that water coexist for a long time.
The water-tree phenomenon finally induces the breakdown of insulation. As the water-tree diagnosis technology, an insulation diagnosis system or the like is known (Refer to Patent Document 1): The insulation diagnosis system carries out an insulation diagnosis for lightning arresters and power cables during power systems are shut down.
In order to select the material for the insulating portion of the insulating cable laid in water or to design the structure thereof, it is necessary to understand various things in advance, including how water tree could occur. Accordingly, it is necessary to carry out a test in which water trees are generated, or reproduced, on a trial basis.
In order to generate water tree, a water electrode method or the like is used. According to the water electrode method, electrode holes are formed in an XLPE plate, which is a test piece. The test piece is then immersed in liquid, and a relatively high electric field is generated near each electrode hole. As a result, water trees are reproduced near each electrode hole of the XLPE.
Usually, in order to form each electrode hole, a mold member which has protruding portions is pushed into the XLPE plate. The electrode holes thus formed could vary each other in shape in many cases. Factors behind the shape variation need to be eliminated before water trees that have been generated around each electrode hole are evaluated. In this manner, the shape variation makes the evaluation of water trees complex.
Pushing the protruding portions into the plate causes damage to the molecular structure of the XLPE at around each electrode hole. As a result, the molecular structure of the XLPE around each electrode hole is different from the molecular structure of other parts of the XLPE. Therefore, it is highly possible that the water trees as those that could emerge in the insulating cable made of XLPE cannot be reproduced exactly by the test using the test piece whose electrode holes have been created by the above-described method.
In view of such circumstances, a technique has been developed to reproduce water trees that are more similar to water trees that could emerge in an actual insulating cable used in liquid (Refer to Patent Document 2).